NEW YORK (Reuters) - Wall Street closed its best month in 20 years on a down note on Monday as the failure of trading firm MF Global Holdings Ltd and new worries about Europe's debt crisis hammered financial shares.

In a sign that Europe's woes were far from over, Italian and Spanish bond yields soared, prompting the European Central Bank to buy the debt, while shares of European banks came under heavy selling pressure.

MF Global Holdings Ltd, the futures broker that made big bets on European sovereign debt, filed for U.S. Chapter 11 bankruptcy protection, making it the biggest U.S. casualty of the euro zone crisis. Trading in MF Global shares was halted.

Financial shares fell sharply. Morgan Stanley, which has tended to do poorly when fears over Europe rise, dropped nearly 9 percent to $17.64. Monday's losses marked a reversal of last week's euphoria over European leaders' deal aimed at containing the debt crisis.

"We started the day with more questions about the European Union," said Mark Grant, Southwest Securities managing director in Fort Lauderdale, Florida.

"Serious questions were raised, and then MF Global came along. MF is involved in all kinds of markets, and the fallout from them going bankrupt is unknown."

As the selloff accelerated at the market's close, the CBOE volatility index jumped over 22 percent, its biggest daily gain since mid-August.

Contributing to the downward pressure, the U.S. dollar shot up to a three-month high against the yen as the government of Japan intervened to curb its currency's appreciation, which hurt the export-based economy.

The jump in the dollar caused shares in energy and natural resources companies to fall sharply. The S&P energy index fell 4.4 percent and was the worst hit sector.

Despite the declines, the benchmark S&P 500 index was up nearly 11 percent for the month and posted its best monthly percentage gain since December 1991.

Most of that run came as European leaders moved to beef up the region's bailout fund and recapitalize its banks. But despite October's gains the S&P 500 index is flat for the year so far.

Still, many analysts believe that with a worst case scenario in Europe seemingly off the cards -- at least for the time being -- stocks could gain further as investors turn their attention to stronger-than-expected economic data in the United States and China.

Bruce Bittles, chief investment strategist at Robert W. Baird & Co in Nashville, said the market's strong advance over the past month was leading to some selling but said the market would likely rise further, provided the S&P 500 held the top end of its recent trading range at around 1,250.

"The market had a huge run in October, so the market was overbought coming into today," he said.

Banks stocks were among the worst performing, with the KBW bank index down 4 percent, although analysts said MF Global was unlikely to be big enough to spark a systemic failure in the banking sector.

JP Morgan Chase, which, according to an MF Global court filing, has about $1.2 billion worth of claims on the brokerage, fell 5.2 percent to $34.76.

The higher greenback pressured commodity prices, with copper off 2 percent and Brent crude 0.3 percent lower. Many commodities are priced in the greenback, making a spike in dollar prices more expensive for traders in other currencies and sapping demand.

"After a solid month of gains, the (higher) dollar is giving traders a reason to shy from the risk trade and take some profits," said Peter Cardillo, chief market economist at Rockwell Global Capital in New York.

Volume was moderate, with about 7.5 billion shares traded on the New York Stock Exchange, NYSE Amex and Nasdaq. Declining stocks outnumbered advancing ones on the NYSE and the Nasdaq by about four-to-one.